A student conducts an experiment to test how music affects plant growth. In the experiment, she uses two identical plant seedlings.

One plant is exposed to classical music. The other plant is exposed to no music. She measures and compares the different plants' height each week, for two months of time.

What is the independent variable in this experiment?

A. two months of time
B. height of the plants
C. type of plant seedlings
D. exposure to music